1. Field of the Invention
This invention relates to a writing instrument such as a felt-tip pen, a marker including a white board marker, or the like, and more particularly to a writing instrument having an ink supplying mechanism which supplies liquid ink directly stored in an ink reservoir to the writing tip of the writing instrument by such a required amount that is used in each writing.
2. Description of the Related Art
In the conventional writing instrument such as a felt tip pen, ink enters and is held due to capillary attraction in the spaces between fibers of a fibrous member filled in the ink reservoir in the barrel of the pen. With this conventional pen, however, the amount of ink held in the ink reservoir is small and the flow rate of ink supplied from the fibrous member to the writing tip such as a felt tip is extremely small. Thus, ink is used up very shortly and becomes faded when the writer writes quickly with the pen. In order to supply ink from the fibrous member to the writing tip, the capillary force of the writing tip which absorbs the ink must be larger than the capillary force of the fibers of the fibrous member. Thus, only the writing tip which has a large capillary force, such as a felt tip or the like, is used.
To overcome these disadvantages, there has been developed a writing instrument in which liquid ink is directly filled in an ink reservoir without using a fibrous material and a slide plug is provided in the pen barrel such that the slide valve divides the interior of the pen barrel into an air region and an ink region and moves toward the ink region as the ink is used.
With the pen of this direct filling type, a large amount of ink can be contained in the ink reservoir, the flow rate of ink supplied to the writing tip is fundamentally not restricted, and the capillary force which the writing tip absorbs ink is not limited either. Thus, this pen has an advantage that a ball point tip for aqueous ink which has a small capillary force or any other type of writing tip can be used.
However, the pen of direct filling type requires a mechanism which supplies, from the ink reservoir to the writing tip, the amount of aqueous ink corresponding to the amount of ink used at the writing tip upon writing. This ink-supply controlling mechanism must be designed such that it supplies to the writing tip a very small amount of ink which correspond to the amount of ink used at the writing tip upon writing and, when the temperature and/or the atmospheric pressure changes, the mechanism prevents excessive pushing out of the ink reservoir from the writing tip (which causes the dripping of ink) and reversely absorption of air into the ink reservoir through the writing tip. The ink supplying mechanisms of this kind are broadly classified into a differential pressure type and a pump type.
The ink-supply controlling mechanism of differential pressure type is provided between the writing tip and the ink reservoir with a valve mechanism which opens when a predetermined pressure difference occurs. More specifically, when ink is used at the writing tip upon writing, the writing tip absorbs ink from the ink reservoir due to the capillary force of the writing tip and a pressure difference occurs between the writing tip and the ink reservoir. When the pressure difference becomes equal to the predetermined value or more than that, the valve mechanism opens, whereby a proper amount of ink is supplied from the ink reservoir to the writing tip. In case writing is not made, the valve mechanism is closed to interrupt communication between the ink reservoir and the writing tip in such a manner that ink is protected from dripping from the writing tip even when the ink in the ink reservoir is expanded or air is prevented from being absorbed in the ink reservoir through the writing tip even when the ink in the ink reservoir shrinks. The expansion and shrinkage of ink in the ink reservoir is compensated by the movement of the slide plug.
The writing tip of the writing instrument having the ink-supply controlling mechanism of differential pressure type must have at least capabilities of opening the valve mechanism of the ink-supply controlling mechanism and causing the slide plug to slide against its sliding resistance. Actually, some ball point tips for aqueous ink lack an ink-absorbing force if they are not suitably designed. In order for such writing instrument have a stable writing characteristic, only a felt to having a high ink-absorbing force can be used.
Quick-drying alcoholic ink is used for a white board marker or the like. Recently, quick-drying alcoholic ink has been developed used in a writing instrument with which writing is made on paper. Such alcoholic ink, however, has a poor wetting characteristic on the surface of an object on which writing is made and sometimes too small a capillary force to provide a sufficient ink-absorbing force.
When the ink-supply controlling mechanism of pump type is employed, the writing tip is adapted to slide along the pen barrel. The controlling mechanism has a pump mechanism, provided in the pen barrel, for supplying a predetermined amount of ink from the ink reservoir to the writing tip due to the sliding of the writing tip. When a writing pressure is exerted on the writing tip upon writing, the writing tip is moved such that ink is pushed out from the ink reservoir to the writing tip by means of the pump mechanism.
This pump type mechanism operates in a simple way and does not require the capillary force of the writing tip. Thus, the writing instrument provided with this mechanism has an advantage that the kind of writing tip and ink is not limited. However, the ink used in one time of writing is very small and it is difficult to design the pump mechanism which supplies this small amount of ink accurately.
Since the pump mechanism is operated in accordance with the movement of the writing tip caused by the writing pressure, the amount of ink coming out of the writing tip differs from a writing pressure to a writing pressure. Some writers write small letters with a strong touch and some others write large letters with a week touch. In the former case, an amount of ink more than that used upon writing is supplied to the writing tip. As the writing is continued, an excess amount of ink is supplied to the writing tip and ink drips therefrom. In the latter case, on the other hand, an amount of ink less than that used upon writing is supplied to the writing tip is delivered by the pump mechanism to the writing tip. As the writing is continued, an insufficient amount of ink is supplied to the writing tip and ink becomes faded.
Both conventional ink-supply controlling mechanisms are provided in the pen barrels, they must be rendered compact and manufactured at a low cost when the are used in throwaway writing instruments. Thus, it is difficult to develop an ink-supplying mechanism which satisfies all the above-mentioned requirements so long as any of the conventional ink-supply controlling mechanisms is employed.